Recently Zazzle had had the unpleasure of telling me that one of my Zazzle products was not acceptable.  In short, it infringed copyright issues - as informed by the relevant licensing company.  I was most surprised: I had no idea that I could not use the words of Oscar Wilde (in/on products for sale).  So that sent me on a useful search that led me to discover basic copyright facts as per the international Berne Convention for the Protection of Literary and Artistic Works (short article).